The present invention concerns an improved process for the preparation of monoesters of polyhydroxy alcohols and in particular for the preparation of polyhydroxy alcohol ester mixtures having high monoester contents. Such materials find application, for example, in the petroleum industry as additives for lubricants and fuels, or in the cosmetics or foods industries.
Fatty acid esters of polyhydroxy alcohols are well known in the art. Such materials may principally be manufactured by two routes:
a direct (xe2x80x98esterificationxe2x80x99) reaction between acid and polyhydroxy alcohol
transesterification of a fatty acid ester with the polyhydroxy alcohol.
In both routes, the product of reaction is typically a complex mixture of esters, a result of the number of possible esterification sites on the alcohol and complex equilibria between individual ester isomers and between mono- and poly-esters in the product mix.
For certain applications, particular ester structures are particularly sought. In the petroleum industry, fatty acid monoesters of polyhydroxy alcohols such as pentaerythritol, sorbitol and particularly glycerol are particularly effective in certain applications. There thus exists a continual need for processes which preferentially synthesise the desired ester(s) to the extent that expensive post reaction separation processes become unnecessary.
U.S. Pat. No. 2,789,119 describes a process for the preparation of monoglycerides for naturally occurring fatty oils, fats or artificially prepared esters, involving the use of tertiary butyl alcohol as reaction medium. The reaction is performed at temperatures of up to 40xc2x0 C. in the presence of alkaline catalyst and results in apparent monoester purities of up to 91% (Example 2) after several days reaction time. Following reaction, the desired monoester is removed by crystallisation where the physical properties of the ester permit. Unreacted glycerol is removed by water washing, the glycerol being extracted by and removed with the aqueous phase.
As the examples of U.S. Pat. No. 2,789,119 indicate, separation of the desired monoesters by crystallisation is a lengthy process, taking several days (Examples 2 and 3). Although a high yield is obtained, the process is thus uneconomic. Higher purities of the monostearate were also obtained relative to the more soluble monooleate ester.
There remains a need in the art for a process which proceeds on an economic time scale and which permits the preparation of monoesters of polyhydroxy alcohols, particularly monoesters of unsaturated acids and polyhydroxy alcohols such as glycerol, to both high yields and purities.
The present invention accordingly provides a process for the preparation of monoesters of one or more polyhydroxy alcohols, comprising:
characterised in that
in the reaction (i) the polyhydroxy alcohol is present in mass excess relative to the glyceride ester composition: the mass ratio of reaction medium to polyhydroxy alcohol is at least 0.8:1 in the case of tertiary butanol or at least 1.3:1 in the case of tertiary amyl alcohol; the reaction is carried out at a temperature at least equal to the reflux temperature of the reaction medium and at a pressure exceeding 1 bar; and the catalyst is selected from basic salts of Group I and Group II metals and non-metallic nitrogenous bases.
The process of this invention provides a high yield of the desired monoester to high purity in the product mix. In particular, it avoids the lengthy crystallisation period of U.S. Pat. No. 2,789,119 through the use of an excess of the polyhydroxyalcohol reactant. The preferred temperature and pressure conditions, and choice of catalyst, also contribute to achieving high yields in an economic reaction time.
High yield processes for the preparation of monoesters also exhibit the secondary problem of emulsification of glycerol liberated from the glyceride ester reactant or, if glycerol is also used as the polyhydroxyalcohol reactant, from unreacted excess starting material. Monoesters of polyhydroxyalcohols show a marked tendency to emulsify glycerol within the ester product phase. As the yield of monoesters increases, this secondary problem also increases to the point where significant glycerol remains within the ester phase and cannot be removed by the conventional water washing technique expounded in U.S. Pat. No. 2,789,119. Where glycerol also comprises the polyhydroxyalcohol reactant, recycling of the glycerol is also rendered less economic by the need to recover it from a water phase.
In a preferred embodiment, the process of the present invention is further characterised by the additional feature of the alcohol removal step (iv) being conducted by low residence time evaporation, and preferably by thin film evaporation. This evaporation step permits the effective removal of excess glycerol, reducing the associated debits of free glycerol in the ester product without the associated emulsification from a water wash.
The process will now be described in more detail, as follows.
The Process Conditions
In the process of the invention, the polyhydroxyalcohol is preferably present in considerable excess, as measured by the mass ratio of alcohol to glyceride ester being at least 1.5:1, and preferably at least 2:1.
A minimum quantity of reaction medium (tertiary butanol or tertiary amyl alcohol) is required relative to the quantity of polyhydroxyalcohol, to ensure homogeneity and maximal yield of monoester. The selection of reaction medium (ie. tertiary butanol or tertiary amyl alcohol) depends on practical factors such as solvency and handleability at ambient temperature.
The reaction (i) is preferably carried out at temperatures exceeding 90xc2x0 C., more preferably exceeding 125xc2x0 C., such as exceeding 150xc2x0 C., and most preferably exceeding 170xc2x0 C. The reaction is preferably carried out at pressures exceeding 3 bars, and more preferably at pressures exceeding 7 bars.
The catalyst for reaction (i) is chosen from the following basic catalyst groups:
1) basic salts of Group I and Group II metals, such as lithium, sodium, potassium, calcium and magnesium salts of, for example, carbonate, hydroxide, oxide, alkoxide or acetate anions. In particular, the catalyst may be selected from lithium carbonate, lithium hydroxide, sodium carbonate, sodium methoxide, potassium t-butoxide, calcium oxide, sodium hydroxide and sodium acetate;
2) non-metallic nitrogenous bases, preferably selected from DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), tetraalkylammonium hydroxides, choline hydroxide, choline bicarbonate, and other strong nitrogenous bases. Other non-metallic bases have been found to be less effective, whilst DBU is most preferred base.
Neutralisation of a catalyst from Group 1 above will usually be necessary. However, catalysts from Group 2 above may be employed without neutralisation and recycled to allow repeated usage. DBU is particularly advantageous in this respect.
The choice of catalyst is important to the process. In particular, other catalysts such as acidic catalysts will lead to re-equilibration of the ester product mixture and loss of high monoester content.
In the preferred embodiment of the process, the thin film evaporation step is preferably conducted at a temperature of 160 to 200xc2x0 C. and, preferably, at a reduced pressure of 0.5 to 40 mbar. A short residence time is preferred to minimise the possibility of ester decomposition in the evaporator.
The Reactants
In all embodiments of the process, the term polyhydroxy alcohol is used to describe a compound having more than one hydroxy group. It is preferred that the polyhydroxy alcohol has at least three hydroxy groups and more prefered that the alcohol be removable from the product mixture by distillation.
Example of polyhydroxy alcohols having at least three hydroxy groups are those having 3 to 10, preferably 3 to 6, more preferably 3 to 4 hydroxy groups and having 2 to 90, preferably 2 to 30, more preferably 2 to 12 and most preferably 3 to 4 carbon atoms in the molecule. Such alcohols may be aliphatic, saturated or unsaturated, and straight chain or branched, or cyclic derivatives thereof. Saturated, aliphatic, straight chain alcohols are preferred.
Advantageously, the polyhydroxy alcohol is glycerol or trimethylol propane. Most preferably, the alcohol is glycerol.
The glyceride ester reactant composition is preferably derived from a natural source such as a vegetable oil. Preferred vegetable oils are triglycerides of monocarboxylic acids, for example acids containing 10-25 carbon atoms, and typically have the general formula shown below 
where R is an aliphatic radical of 10-25 carbon atoms which may be saturated or unsaturated.
Generally, such oils contain glycerides of a number of acids, the number and kind varying with the source vegetable of the oil.
Examples of oils are rapeseed oil, coriander oil, soyabean oil, cottonseed oil, sunflower oil, castor oil, olive oil, peanut oil, maize oil, almond oil, palm kernel oil, coconut oil and mustard seed oil. Rapeseed oil, which is a mixture of fatty acids partially esterified with glycerol, is preferred as it is available in large quantities and can be obtained in a simple way by pressing from rapeseed.
Alternatively, the glyceride esters may be derived from an animal source such as beef tallow oils or fish oils.
The glyceride ester composition, and in particular the composition derived from a vegetable source, comprises triglycerides of saturated and unsaturated fatty acids with 12 to 22 carbon atoms, for example of lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, elaidic acid, petroselic acid, ricinoleic acid, elaeostearic acid, linoleic acid, linolenic acid, eicosanoic acid, gadoleic acid, docosanoic acid or erucic acid, which have an iodine number from 50 to 150, especially 90 to 125. Oils predominating in glycerides of one or more of oleic, linoleic, linolenic and erucic acids are preferred.
The invention will now be described by way of examples as follows: